The present invention relates to a closed cell foam of ethylene vinyl acetate copolymer (EVA) and more particularly to an electrically conductive plastic foam of EVA and method of making thereof.
According to the volume resistivity, materials can be classified as electrically conductive materials and insulating materials. The volume resistivity of conductive materials is from 0 to 100K ohm cm. The volume resistivity of insulating materials is no less than 200K ohm cm. Most of conventional plastics are insulators, and very few of plastics are electric conductors. The electric and thermal insulations of plastic foam of EVA are higher than those of conventional plastics. There are two kinds of plastic foams, namely, open cell foam and closed cell foam. It is difficult to manufacture an electrically conductive open cell foam, and it is more difficult to manufacture an electrically conductive closed cell foam such as a closed cell foam of EVA. In general, the volume resistivity of a closed cell foam is higher than that of an open cell foam of the same material. Therefore, it is easier to manufacture an open cell foam of EVA which is electrically conductive. The volume resistivity of the open cell foam of EVA is approximately 100K ohm cm. It is very difficult to manufacture an open cell foam of EVA which has the volume resistivity no more than 50K ohm cm. Therefore, it is more difficult to manufacture a closed cell foam of EVA which has the volume resistivity no more than 50K ohm cm.